1. Field of the Invention
This invention relates to liquid crystal displays. More particularly, it relates to liquid crystal displays that use ferroelectric and anti-ferroelectric liquid crystals.
2. Discussion of the Related Art
A conventional liquid crystal display (LCD) includes a display panel having upper and lower substrates that are attached to each other, and an interposed liquid crystal, usually a nematic, a smetic, or a cholesteric liquid crystal. A liquid crystal display utilizes the electro-optic effects of the liquid crystal. The display panel is operationally divided into a plurality of liquid crystal cells. On the exterior surfaces of the upper and lower substrates, polarizers or retardation films are selectively attached.
A major design consideration of a liquid crystal cell is the characteristic of the particular liquid crystal that is used. A good liquid crystal should have a fast response time, a good gray scale, and a wide viewing angle, all while operating at a low driving voltage. However, it is very difficult to find a liquid crystal that has all of those characteristics. Thus, various designs have been adopted for liquid crystal display devices.
Among the various types of liquid crystals displays, the 90° twist mode TN liquid crystal display has neither a wide viewing angle nor a fast response time. An in-plane switching (IPS) mode has a wide viewing angle, but a relatively slow response time that results in decreased luminance and poor moving image quality. To overcome the problem of a slow response time, various technologies have been proposed. For a fast response time, LTN (low twisted nematic) and OCB (optically compensated birefringence) modes have been studied. However, those technologies have not yet been able to provide the fast response time of a CRT.
FIG. 1 is a cross-sectional view illustrating a conventional TN-LCD panel. As shown in FIG. 1, the TN-LCD panel has lower and upper substrates 2 and 4 and an interposed liquid crystal layer 10. The lower substrate 2 includes a substrate 1 having a TFT “S” that is used as a switching element that changes the orientation of the liquid crystal molecules. The TFT “S” includes a pixel electrode 14 that applies a voltage to the liquid crystal layer 10 in accordance with signals that are applied to the TFT “S”. The upper substrate 4 has a color filter 13 for implementing color, and a common electrode 12 on the color filter 8. The common electrode 12 serves as an electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 14 is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the liquid crystal layer 10 between the two substrates 2 and 4, those substrates are sealed by a sealant 6.
FIGS. 2A to 2C illustrate possible alignments of liquid crystal molecules in a liquid crystal layer. In a nematic liquid crystal, while each rod-like molecule fluctuates quite rapidly, as shown in FIG. 2A the molecules have a definite orientational order expressed by a unit vector “ñ” called a director. As shown in FIG. 2B, in a smetic liquid crystal the molecules positionably have a layered structure in which the molecular orientation is tilted perpendicular to the layers. As shown in FIG. 2C, in a cholesteric liquid crystal, the director ñ changes its orientation gradually along a helical axis. The helical axis coincides with the optical axis of this material. Among the different types of liquid crystals, the nematic liquid crystal is most widely used in liquid crystal display devices.
Liquid crystals for liquid crystal display devices should:                a) have a liquid crystal phase that extends from low to high temperatures, and thus can operate over a range of temperatures;        b) be chemically and optically stable over time;        c) have a low viscosity and a fast response time;        d) have highly ordered molecular alignments, and thus provide good contrast; and        e) have a large dielectric anisotropy and a low operating voltage.        
The electro-optic effect enables electrical modulation of light by changing the alignment of the liquid crystal molecules using an applied electric field.
Among the various types of nematic liquid crystals, a twisted nematic (TN) liquid crystal and a super twisted nematic (STN) liquid crystal are often used. In a TN liquid crystal panel, a nematic liquid crystal is interposed between transparent lower and upper substrates. Those substrates induce a definite molecular arrangement such that a gradual rotation of the molecules occurs between the lower transparent substrate and the upper transparent substrate until a twist angle of 90 degrees is achieved. In an STN liquid crystal panel the angle of twist rotation is increased to 180 or to 360 degrees.
The basic configuration and operation of a twisted nematic liquid crystal display device will now be explained. As shown in FIG. 3A, opposed and spaced apart first and second polarizers 10 and 16, respectively, have perpendicular first and second transmittance axis directions 40 and 42. Between the two polarizers 10 and 16 are first and second transparent substrates 12 and 14, which are also opposed to and spaced apart from each other. Spacers are used to maintain the cell gap between the substrates. For example, plastic balls or silica balls having a diameter of 4 to 5 micrometers can be sprayed on the first substrate.
Still referring to FIG. 3A, the first and the second transparent substrates 12 and 14 include first and second orientation films 20 and 22, respectively, on their opposing surfaces. Between the first and second orientation films 20 and 22 is a positive TN liquid crystal 18.
The positive TN liquid crystal has a characteristic that it arranges according to an applied electric field. The first and second polarizer 10 and 16, respectively, transmit light that is parallel with their transmittance-axis directions 40 and 42, but reflect or absorb light that is perpendicular to their transmittance-axis directions 40 and 42.
The first and second orientation films 20 and 22 were previously rubbed in a proper direction with a fabric. This rubbing causes the positive TN liquid crystal molecules between the first and second transparent substrates 12 and 14 to become tilted several degrees. First and second rubbing directions 50 and 52 of the first and second orientation films 20 and 22 are, respectively, parallel with the transmittance-axis directions of the first and second polarizers 10 and 16. With no electric field applied across the positive TN liquid crystal 18, the orientation of the liquid crystal molecules twists between one substrate and the other at a definite angle, that angle being the twisted angle of the positive TN liquid crystal 18.
During operation, a back light device 24 irradiates white light onto the first polarizer 10. The first polarizer 10 transmits only the portion of the light that is parallel with the first transmittance-axis direction 40. The result is a first linearly polarized light 26 that passes through the polarizer 10. The first linearly polarized light 26 then passes into the positive TN liquid crystal 18 via the first transparent substrate 12.
As the first polarized light 26 passes through the positive TN liquid crystal 18, the first polarized light 26 changes its phase according to the twist alignment of the positive TN liquid crystal molecules. Accordingly, the first linearly polarized light 26 becomes an elliptically (possibly circularly) polarized light 28.
The elliptically polarized light 28 passes through the second transparent substrate 14, and meets the second polarizer 16. When the elliptically polarized light 28 passes through the second polarizer 16, the second polarizer 16 transmits only the portion of the elliptically polarized light 28 that is parallel to the second transmittance-axis direction 42. A polarized light 30 is then emitted. In the above-mentioned operation, a white state is displayed.
Turning now to FIG. 3B, when a voltage supplier 35 induces an electric field through the positive TN liquid crystal 18, the positive TN liquid crystal molecules rotate and arrang such that the longitudinal axes of the molecules become perpendicular to the surfaces of the first and second substrates 12 and 14. Accordingly, the first linearly polarized light 26 passes through the first transparent substrates 12, the positive TN liquid crystal 18, and the second transparent substrate 14 without phase change. The first linearly polarized light 26 then meets the second polarizer 16. As the second polarizer 16 has the second transmittance-axis direction 52 which is perpendicular to the first linearly polarized light 26, the second polarizer 16 absorbs or shields most of the first linearly polarized light 26. Thus, little or none of the first linearly polarized light 26 passes through the second polarizer 16. Accordingly, a dark state is displayed.
Generally, the response time of a nematic liquid crystal display is greater than 15 ms. This is because of a low elastic constant and the time required to polarize the molecules. This results in decreased luminance when a nematic liquid crystal display produces high speed moving images.
Because of that limitation, a ferroelectric liquid crystal (FLC) in the smetic phase has become of interest. The FLC has a hundred times faster response time than the TN LC or the STN LC. This is because the FLC has a spontaneous polarization that leads to high-speed responses, and thus an improvement in the display of moving images. For example, a deformed helix FLC (DHF), a surface stabilized FLC (SS-FLC), an AFLC, a V-shape FLC, and a half-VFLC (HV-FLC) have been developed for the LCD device.
Except for the DHF FLC, to use a ferroelectric LC in a liquid crystal display application, the cell gap between the two transparent substrates of a liquid crystal display device should be uniformly maintained at less than about 2 micrometers.
Among the various ferroelectric LCs, the V-shape FLC and the HV-FLC have recently become of significant interest due to their advantages in producing gray scale images and to their operational modes. Prototypes implementing these FLCs have been produced.
FIGS. 4A and 4B respectively illustrate electro-optic properties of the V-shape FLC and the HV-FLC. As indicated, the transmittance of the V-shape FLC and the HV-FLC are readily controlled by a voltage.
When driving a TFT Liquid Crystal Display, the VHR (voltage holding ratio) property is important. As the VHR becomes smaller, each pixel element requires a larger storage capacitance so that more charge becomes available to prevent or to compensate for charge leak. However, as shown in FIG. 5 this decreases the aperture ratio. Additionally, a larger storage capacitor requires a higher voltage driver IC.
In general, for a nematic liquid crystal TFT-LCD the VHR is above 95%. Thus, the VHR it is not a significant problem for such displays. However, a ferroelectric liquid crystal (FLC) has a VHR that is much smaller, possibly under 50%. This is due to the depolarization effect of the liquid crystal, which is directly related to spontaneous polarization. So, as shown in FIG. 6, the VHR becomes smaller as the spontaneous polarization (Ps) becomes larger.
As result, It is difficult to drive a FLC having a large spontaneous polarization with a TFT
The Ps (spontaneous polarization) value of a V-shape FLC ranges into several hundred nC/Cm2, but an HV-shape FLC has a Ps value under 10 nC/Cm2. So, the HV-FLC has a significant advantage in this regard.
Additionally, the V-shape FLC undergoes phase transitions of “Isotropic-->N*-->Sc*-->crystal” as the temperature decreases. The HV-FLC has phase transitions of “Isotropic-->N*-->Sc*-->crystal” as the temperature decreases. If only cooling is conducted until the Sc* phase (the phase at room temperature and at the driving temperature), random 2-domain molecular orientations appear due to unwanted molecule configurations as shown FIG. 7A.
However, if an appropriate DC (direct current) electric field (−polarity) is applied during the N*-->Sc* phase transition (over a small temperature range), the unwanted molecule configuration is suppressed. As a result, as shown in FIG. 7B, all of the molecules properly align. FIG. 7B shows a proper alignment (mono-domain) achieved using an appropriate DC electric field. In this state, the liquid crystals continuously rotate around a cone angle of 50° until they react to an applied electric field. FIG. 8A illustrates the driving of a HV-FLC, while FIG. 8B illustrates electro-optic curves of the driven HV-FLC.
So, a HV-FLC has good alignment, the same transmittance as a nematic due to the 50° cone rotation, and a small Ps, all of which makes them suitable for LCD applications.
While the HV-FLC has many good properties, is has very weak resistance to both thermal stress and mechanical stress. Although all molecules can be aligned by applying an appropriate DC (direct current) electric field (−polarity) during the N*-->Sc* phase transition, thermal stress causes the reverse transition from Sc*-->N* to occur. Then, the mono-domain alignment is broken and the unwanted random domains reappear after cooling back to the Sc* phase. Furthermore, mechanical stress can break the mono-domain alignment. Then, the unwanted random domains reappear.
FIG. 9A illustrates an electro-optic curve and the image of a mono-domain aligned cell. FIG. 9B shows electro-optic curves and an image of a cell having a broken alignment and random 2-domains. As shown, light leakage occurs from a cross polarizer.